build.sh: handle building experimental without version tag

build.sh

@@ -35,7 +35,7 @@ pushd $MY_DIR > /dev/null
 . functions.sh
 
 ### CHECK THAT VERSION DOES NOT YET EXISTS
-[ -x "versions/${VERSION}" ] && abort "There exists a version file for '$VERSION' ... you are trying to do something really stupid, aren't you?"
+[ -n "$VERSION" -a -x "versions/${VERSION}" ] && abort "There exists a version file for '$VERSION' ... you are trying to do something really stupid, aren't you?"
 
 ### INIT /src IF NECESSARY
 if [ ! -d "$CODE_DIR" ]; then